1. Field of the Invention
The present invention relates to the formation of a magnetic pattern on a recording medium such as a magnetic disc. Still further, the invention is directed to the verification of servo patterns placed onto a magnetic disc. The present invention further pertains to a method for detecting missing servo patterns at very fine scales.
2. Description of the Related Art
The computer industry employs magnetic discs for the purpose of storing information. For example, computer systems employ disc drive systems for transferring and storing large amounts of data between magnetic discs and the host computer. The magnetic discs tend to be circular in shape, and are fabricated as a series of discrete layers. The operative layer of any disc is a “magnetic layer” that resides intermediate the series of layers. Because of the thin geometry of the magnetic layer, magnetic information storage discs are sometimes referred to as “thin film magnetic discs.”
FIG. 1 presents a perspective view of magnetic media 10 as are commonly employed for information storage. In this view, a plurality of stacked magnetic discs 10′ is shown. The discs 10′ are shown in FIG. 1 in vertical alignment as is common within a disc drive system. In this respect, each disc 10 has a central concentric opening 5 for receiving a spindle (shown at 51 in FIG. 3). A rotary motor drives the spindle 51, causes the discs 10 of the disc pack 10′ to rotate in unison.
Each disc 10 is typically fabricated from a series of layers that includes at least a substrate and a magnetic layer. The substrate provides the structural integrity for the magnetic medium 10. The substrate may be fabricated from a nickel-phosphorus plated aluminum disc, or other material such as glass or manganese-oxide. The magnetic layer is uniformly applied on the substrate. The magnetic layer is preferably formed of a cobalt-based alloy, such as a cobalt-chromium-tantalum alloy, though other types of magnetic layers are known.
To enhance the durability of the disc 10, an overcoat (not shown) is preferably deposited over the magnetic layer. The overcoat helps reduce wear of the magnetic media 10 due to contact with a magnetic read-write head assembly (not shown in FIG. 1). The overcoat also aids in corrosion resistance for the magnetic media 10. The overcoat preferably is a layer of sputtered amorphous carbon, though other materials such as sputtered ceramic zirconium oxide and amorphous films of silicon dioxide are suitable.
An exemplary magnetic medium 10 may also have a lubricant layer. The lubricant layer (not shown) also assists in reducing wear and corrosion of the magnetic media. The lubricant is preferably a perfluoropolyether-based (PFPE) lubricant having a thickness of 10 to 20 Angstroms.
It should be noted that the present invention is not limited to any particular type of magnetic medium. In this regard, the discs 10 of FIG. 1 and the description above are purely exemplary, and any disc structure may be used for the methods of the present invention so long as the media generates magnetic flux.
During fabrication, the magnetic layer is deposited as a homogeneous layer in both the radial and circumferential directions. At that point, the magnetic layer carries no magnetic charge. After deposition of the magnetic layer, information is magnetically written onto the medium 10. The magnetic information provides numerous aligned magnetic domains in the structure of magnetic layer, allowing data to be read by sensing the alternating direction of magnetization. More specifically, a magnetic read head (not shown in FIG. 1) senses transition locations where the direction of aligned magnetic domains reverses within the magnetic layer. The magnetic transition patterns are known as “servo burst” patterns. These patterns are shown at 26 in FIG. 2, and will be described in greater detail below.
FIG. 2 schematically shows an exemplary portion of servo-pattern information 26 magnetically recorded on a disc 10. Magnetization signs 22, 24 indicate the direction of magnetization from the aligned magnetic domains. In FIG. 2, transition boundaries 27 between areas of opposite magnetic domain alignment are shown in solid lines. It can also be seen that servo patterns are placed side-by-side along essentially concentric tracks 28. The boundaries 25 of each track 28 are shown in small dashed lines, and a centerline 29 of each track 28 is shown in larger dashed lines. It is understood that the boundaries 25 of each track 28 and the centerlines 29 are not recognizable by any physical properties of the magnetic layer 15, but are shown for conceptual purposes only. A servo burst pattern commonly has dimensions of 10 to 20 micrometers in the tangential direction and dimensions of a track width (about 0.1 micrometer) in the radial direction.
In operation, information stored in the magnetic layer of the disc 10 is read by a magnetic head assembly. The magnetic head assembly is part of a disc drive system, such as the system 50 shown in FIG. 3. FIG. 3 presents a top view of an exemplary disc drive system 50, with the magnetic head assembly seen at 58. The disc drive assembly 50 includes a servo spindle 52 and an actuator arm 54. The servo spindle 52 is motorized to pivot about an axis 40. More specifically, the servo spindle 52 is selectively positioned by a voice coil motor 57 which pivots the actuator arm 54, causing the arm 54 to move through arc 42. In this manner, the arm 54 can be positioned over any radial location “R” along the rotating disc surface.
The actuator arm 54 carries a flexure arm or “suspension arm” 56. The suspension arm 56, in turn, supports the magnetic head assembly 58 adjacent a surface of a disc 10. The head assembly 58 defines a transducer that is capable of reading magnetic information from the magnetic layer of the disc 10, or writing additional information on a reserved portion of the disc 10. The magnetic head 58 is typically placed on a small ceramic block, also referred to as a slider. The slider is aerodynamically designed so that it “flies” over the disc 10 as the disc is rotated at a high rate of speed.
As noted, the disc 10 itself is supported on a drive spindle 51. The drive spindle 51 rotates the disc 10 relative to the magnetic head assembly 58. The disc rotates about axis 45. As the disc 10 rotates, the air bearing slider on the head 58 causes the magnetic head 58 to be suspended relative to the rotating disc 10. The flying height of the magnetic head assembly 68 above the disc 10 is a function of the speed of rotation of the disc 10, the aerodynamic lift properties of the slider along the magnetic head assembly 58, and a biasing spring tension in the suspension arm 56.
It should be noted at this point that the typical disc drive system 50 is able to accommodate multiple discs 10, as shown in the disc stack 10′ of FIG. 1. To this end, the drive spindle 51 receives the central openings 5 of the respective discs 10. Separate suspension arms 56 and corresponding magnetic head assemblies 58 (not shown in FIG. 3) reside above each of the discs 10.
Each disc 10 has a landing zone 11 where the magnetic head assembly 58 lands and rests when the disc drive 50 is turned off. When the disc drive assembly 50 is turned on, the magnetic head 58 “takes off” from the landing zone 11. Each disc 10 also has a data zone 17 where the magnetic head 58 flies to magnetically store or read data.
As noted, the servo spindle 52 pivots about pivot axis 40. As the servo spindle 52 pivots, the magnetic head assembly 58 mounted at the tip of its suspension arm 56 swings through arc 42. This pivoting motion allows the magnetic head 58 to change track positions on the disc 10. Polar coordinates 41 are established based on the geometry of the disc 10. The perpendicular distance from the axis 45 to any location on the disc 10 is defined by a radius R, while the circumferential dimension is denoted by azimuthal dimension 0. An optional vertical distance Z may be provided (shown in FIG. 1) where multiple discs 10 are stacked.
The ability of the magnetic head 58 to move along the surface of the disc 10 allows it to read data residing in tracks (shown in FIG. 2) along the magnetic layer 15 of the disc. Each read/write head 58 generates or senses electromagnetic fields or magnetic encodings in the tracks of the magnetic disc as areas of magnetic flux. The presence or absence of flux reversals in the electromagnetic fields represents the data stored on the disc. The disc drive 50 must be able to differentiate between tracks 28 on the disc 10 and to center the magnetic head 58 over any particular track 28. To accomplish this, most disc drives 50 use embedded “servo patterns” 26 of magnetically recorded information on the disc 10. The servo patterns 26 are read by the magnetic head 58 to inform the disc drive 50 of track location.
The exemplary servo pattern includes “gray code” (shown at 60 in FIG. 2) and “servo burst” (shown at 62 in FIG. 2). The gray code indexes the radial position of the track such as through a track number, and may also provide a circumferential index such as a sector number. The servo burst is a centering pattern to precisely position the head over the center of the track. Each servo burst includes magnetic transitions 64, 66 on the inside of the track 28 interleaved with magnetic transitions on the outside of the track 28. If the magnetic head 58 is centered over the track 28, the signal read from the inside transitions (shown at 64 in FIG. 2) will be equal and opposite to the signal read from the outside transitions (shown at 66 in FIG. 2) If the magnetic head 58 is toward the inside of the track 28, the signal from the inside transitions will predominate, and vice versa. By comparing portions of the servo burst signal 26, the disc drive 50 can iteratively adjust the head location until a zeroed position error signal is returned from the servo bursts, indicating that the head 58 is properly centered with respect to the track 28. Additional details concerning various configurations of servo patterns is presented in U.S. Pat. No. 5,991,104 entitled “Using Servowrite Medium for Quickly Written Servo-Patterns on Magnetic Media.”
Disc drives which magnetically record, store and retrieve information on disc-shaped media are widely used in the computer industry. However, before data can be read from a disc and before a disc can be used in a computer, initial programming and data must be stored on the disc 10. Thus, during manufacturing, servo information is encoded on the disc and subsequently used to accurately locate the transducer 58. In contrast to data sections, servo patterns are written only once and are not written over by the magnetic head during operational use of a disc drive. The servo pattern information, and particularly the track spacing and centering information, needs to be located very precisely on the disc tracks. Most storage discs utilize a multiplicity of concentric circular tracks, though some discs have a continuous spiral forming a single track on one or both sides of the disc.
Various means are known for recording information on a disc 10. In one arrangement, a write transducer, or “servo track writer,” (not shown) is placed on a magnetic head assembly similar to the one shown in FIG. 3. A write transducer is used to record information on the disc 10, including servo patterns. At the time the servo information is written, the disc drive is typically at the head disc assembly stage. The head disc assembly includes most of the mechanical drive components shown in FIG. 3, but does not typically include all the drive electronics and may only read on one disc 10 rather than a disc pack 10′ During the track writing process, the servo track writer precisely locates the transducer heads relative to the disc surface and writes the servo information thereon. Accurate location of the transducer head is necessary to ensure that the track definition remains concentric.
The write transducer creates a highly concentrated magnetic field. During writing, the strength of the concentrated magnetic field directly under the write transducer is greater than the coercivity of the recording medium (known as “saturating” the medium). Grains of the recording medium at that location are magnetized with a direction that matches the direction of the applied magnetic field. The grains of the recording medium retain their magnetization after the saturating magnetic field is removed. As the disc 10 rotates, the direction of the writing magnetic field is alternated based on bits of information being stored, thereby recording a magnetic pattern on the track directly under the write transducer.
More recently, faster and more reliable means have been developed for writing data onto a magnetic disc. These methods include the use of a pre-formatted magnetic stamper that conveys data to an unformatted magnetic disc. Examples of such writing or “stamping” methods are described in U.S. Pat. No. 5,991,104 and U.S. Pat. No. 6,181,492, each of which is entitled “Using Servowrite Medium for Quickly Written Servo-Patterns on Magnetic Media.” The '104 and '492 patents were each assigned to Seagate Technology, LLC. Under these patents, a master disc having a pre-formatted magnetic pattern is placed in close proximity to a “slave disc.” A strong magnetic field is applied to the slave disc to form a uniform direction of premagnetization in the medium. The pre-magnetized medium is then brought into contact with a master for servo printing. The pre-magnetized medium is exposed to a uniform magnetic field through the master, sometimes referred to as a “stamper.” Thus, servo information is transferred via fringing magnetic fields emanating from the pattern previously imposed upon the stamper. The result is that the magnetic pattern from the master is formed in the slave disc.
Using magnetic stamper technology, servo pattern information may be quickly imparted onto a blank disc 10. This technology is sometimes referred to as “servo media printing.”
At the time servo patterns are written, there are no reference locations on the disc surface which can be perceived by the master disc (or a servo track writer); rather, the method of contact servo printed media relies upon the direct transfer of magnetic patterns from the stamper to a disc. For this magnetic transfer of information to be effectuated, an extremely narrow interface between the stamper and the blank disc is required. Stated another way, in order to effectuate servo media printing and to replicate the very fine sub-micron features of the servo pattern, an extremely small interface is required between the stamper and the blank disc. For this reason, the method of contact servo printed media is very sensitive to any disturbances that interfere with the close mechanical relationship. Mechanical defects in the stamper or even unwanted dust particles on the stamper face may interfere with the stamper/disc blank interface. The result of such disturbances is that areas of missing servo pattern may be formed.
It is desirable to ensure that there are no areas of missing servo pattern on the disc. In this respect, magnetic storage of information must be virtually 100 percent error free for a disc to be operational. Therefore, a full surface test is desired for every printed disc.
U.S. Pat. No. 6,373,243 entitled “Magnetic Media Tester for Testing a Servo Signal Prerecorded in a Magnetic Media” issued in 2002. This patent provides a device for testing magnetic media such as a disc. Among other features, the tester comprises a voice coil motor disposed on a rotary positioner, and a pair of magnets sandwiching the voice coil The device allows a magnetic head to be precisely positioned for testing by the same construction as a hard disc drive of the actual machine. In this way, signals can be accurately generated by the magnetic media tester to determine the placement of servo pattern information.
Despite the contribution of the '243 patent, a need exists to conduct a full surface test in a more expeditious manner. A full surface test must possess the properties of high throughput and low cost in order to be compatible with the economic constraints of memory disc manufacturing.